Electron Microscopy is one of the most important of the shared services at the Jackson Laboratory. Nearly 40% of the research staff has, at one time or another, made use of this service. It is also used by scientists at nearby Mount Desert Island Biological Laboratory and by scientists working at the Jackson Laboratory as Visiting Investigators from other institutions. The Laboratory is strongly committed to maintaining this service. The Electron Microscope Suite occupies approximately 1,450 square feet and consists of separate rooms housing the scanning and transmission electron microscopes. There are, in addition, two microtome rooms, three dark rooms and a large preparation room. The transmission EM in current use is a Hitachi HU-11C purchased in 1969. Since the Laboratory also has a JEOL JSM 35C scanning EM, purchased in 1980, and is very well satisfied with its performance and servicing, we are seeking to replace the old Hitachi with a state-of-the-art JEOL JEM-100CX. Addition of a JEM-100CX should immediately increase the use of the Electron Microscopy Service by 20% by adding an entirely new user group of molecular biologists. The Hitachi is presently of very limited use to this group and they are anxious to pursue DNA-DNA heteroduplex studies and examination of chromosomal aberrations with the JEM-100CX. All users will profit from the reduced specimen contamination, wider range of magnification and the increased ease of operation of the JEM-100CX. In addition, the "Mini-Lab" feature provides for expandability by providing a vacuum chamber (above the objective lens) in which various accessories may be installed. Without an updated transmission electron microscope at the Jackson Laboratory, many investigators would be forced to send material, or travel themselves, to institutions with the necessary equipment. This would prove costly and time consuming and would delay research considerably.